1. Field of the Invention
This invention relates generally to steam turbine control valves, and more particularly to assemblies used in actuating such steam turbine control valves.
2. Statement of the Prior Art
Admission of steam to steam turbines is typically controlled by way of a plurality of control valves which is installed in steam chest portions of the steam turbine. In prior art steam turbine control valve systems, each control valve is actuated open and closed by various suitable means including pneumatic, hydraulic, and electrohydraulic means. The stems of such control valves are, in most cases, raised to open and lowered to close the control valve mechanically through linkages including a lever.
For example, FIGS. 1 and 2 illustrate one typical actuator means that is presently used for opening/closing a steam turbine control valve. The control valve 10 is shown attached, through its valve stem 12, to a lever 14 point A. One end of lever 14 is rotatably coupled to a portion 16 of the steam chest of a conventional steam turbine (not shown) by a first link 18, while the other end of the lever 14 is rotatably coupled to a reciprocable piston rod 20 through a second link 22. As is well known, the piston rod 20 forms part of a conventional servomechanism 24 which actuates the control valve 10 between its closed position as is shown in FIG. 1 to an open position shown in FIG. 2. Servomechanism 24 typically includes an actuator 26 to push the piston rod 20, and a control block manifold 28 to control the actuator 26.
One problem that is frequently encountered in using such lever-operated steam turbine control valves is the "side forces" that may impact upon the piston rod 20 throughout its stroke. When the control valve 10 is closed as shown in FIG. 1, there are no such side forces which affect the piston rod 20 because its actuating forces are at rest. However, as the control valve 10 moves towards an open position shown in FIG. 2, resultant side forces F.sub.side are imposed on the piston rod 20 due to the force F.sub.link acting off of the vertical. The magnitudes of these side forces F.sub.side are proportional to the angle from which the second link 22 is offset from the piston rod 20.
Another problem that is also frequently encountered in using such lever-operated steam turbine control valves are the large moments and resulting forces that are placed on the actuator and its support assembly. Since these actuators are typically mounted upon the side of the control valve body, in the manner shown in FIGS. 1 and 2, they require not only many extensive and often inaccurate support analyses, but also the use of retrofitted, adjustable jackscrews to ensure that they are properly supported. These forces cause large deflections of the support assemblies from their respective control valve bodies upon which they are mounted. Furthermore, the control valve themselves are subjected to a great deal more heat than the support assemblies which are mounted thereon. It will be appreciated, therefore, that any adjustment of the stiffening jackscrews will be difficult because the control valve bodies will expand significantly more than their support assembly.
A related problem to these large moments and forces is the concomitant requirement to manufacture larger, stronger and more complex support assemblies to counteract such moments and forces. More often than not, these support assemblies are made of cast components. A simpler manufacturing design would be more desirable.